Just like the weather, about which everybody jokes but against which no one ever takes action, there’s another equally uncaring adversary. It’s one against which we are all totally and utterly helpless, and each time we proudly mark the mounting hours in every succeeding line of our logbooks, we must first reconcile its sepulchral tally. Our opponent is the Hobbs meter.
As your flight instructor probably pointed out during your first cockpit familiarization, along with all the other instruments in your cockpit is this little set of numbered wheels behind a small window (circular or rectangular) that reads off tenths of hours, hours, tens, then hundreds, then thousands, usually whenever the engine is running. The old Hobbs meter is what the FBO probably uses to bill you, as well as keep tabs on when the next “hundred hour” inspection is due.
If you’re renting your aircraft from an FBO, you can know with the certainty of death and taxes that every tenth-hour digit on the far right of its display is setting you back a few dollars. When I rented a Piper Cub at the grass field near my home, we didn’t have a Hobbs meter; we just used our watches and the honor system. But each time the big hand went another six ticks around it meant that I was out another four bucks. (Ah, those were the days!) When I rent a Robinson helicopter, you can bet there’s a Hobbs in there, and that six minutes becomes almost $30. Get a checkout in a King Air C90 twin turboprop, and each six-minute digit might run you roughly $100! That certainly does confer a new perspective on going once around the pattern.
Almost everywhere I’ve rented aircraft in the Mid-Atlantic area, flight time is usually reckoned by the reading on the Hobbs meter. But there’s another, more merciful variant; if you’re in a flying club, your flight time may be tallied by the tachometer. These two types of meters actually keep time quite differently. Being aware of how each functions won’t ease all the pain, but it might mitigate some puzzlement.
The oil pressure gauge is important to the operation of the Hobbs meter. Actually, for most airplane Hobbs meter installations, oil pressure is what makes it start “ticking.” There’s usually a wire from ground to one terminal connector of the Hobbs meter, and a wire from its other terminal then goes to a pressure sensor or switch, and another wire completes the circuit to the power supply. When oil pressure rises beyond some predetermined minimum value—usually about 10 pounds per square inch absolute—the switch contacts close, the circuit is completed, and at that point, your wallet becomes eligible for a new weight-and-balance. The meter reckons time at the same speed as your wristwatch, whether you’re climbing out at full power or sitting with the engine idling at 950 rpm and number three for takeoff. You can’t save money by shutting off the master switch, because it’s almost never connected through it. (You could shut down the engine, but that might make the folks behind you nervous....)
Not all Hobbs meters are connected to an oil pressure switch. Some airplanes have an “airspeed” switch, which basically only counts flight time. (Like the oil pressure switch, it activates when reaching a particular airspeed, usually takeoff speed.) Some complex airplanes use the squat switch or gear retraction, basically accomplishing the same thing, which would imply you could taxi until the cows came home and the owner wouldn’t care—but don’t bet on it. Some are wired to the ignition switch. And another, less common method is when the Hobbs meter is connected to the battery master switch, which means, of course, that you could be paying even if the engine isn’t running.
Now we come to the tachometer. In addition to an analog dial showing the engine rpm, there often are digits within its face, having a presentation much like the Hobbs meter. The traditional ones are mechanical, although many are digital, and some don’t record hours at all. Those that do usually don’t mark the passage of time like the Hobbs meter.
With the tach-based hour meter, there’s a geared drive from the registered rpm, connected to the engine via cabling, but it’s made to keep “true time” only at one particular engine rpm, which in a typical trainer at high cruise might be, say, 2,500. If you’re rolling down a two-mile taxiway at Big City International at 1,250 rpm, that slower rpm tach time will only register six minutes for every 12 minutes of actual clock time. And when you’re sitting there behind three other airplanes, idling at 800, stewing about the long wait—well, if you’re paying by tach time, things could be worse.
If you’re flying along on your cross-country flight at 2,100 rpm, or doing pattern work at 2,000, then it’s a bit like relativity: Time would pass more slowly for you—and your wallet. Flying clubs often bill their members by tach time, where you ring up less time than you actually flew, because all they really need to do is make ends meet. An FBO bills by Hobbs time because not only does it want to make ends meet, but it wants to make a profit as well. (If you own your own airplane, the only concern you might have is perhaps flying at that unpublished “best miles per gallon” airspeed.)
When flying an airplane that you rented from the FBO, your natural inclination will probably be to use a higher power setting that makes the tach hour nearly equal a clock hour, because that’s what you’re going to get billed for, anyway. For most trainers, if flown at conservative power settings, the difference between Hobbs and tach time might be as much as 20 percent (with the latter being the smaller of the two). If you pay by the tach hour, determining the most economical “best tach speed” for cross-country flight might be a useful exercise.
Some operators ask their renter pilots to write down the start and end times for both Hobbs and tach hours. This helps alert their maintenance folks so that any malfunction in either instrument will be more easily noticed, and keeps the accountability trail as current as possible. For example, it’s theoretically possible for congealed oil to hold the Hobbs switch open after an unsuccessful engine start on a cold morning. If your rental log can prove it was at such-and-such when you first got there, no one can point an accusing finger at you.
Obviously, before you fly any airplane that isn’t yours, cover yourself by making sure that what the previous person wrote down for an ending time is what you see when you first look inside the cockpit. It’s a rare pilot who has been flying for awhile who hasn’t caught a bookkeeping error—such as an undocumented maintenance flight, for example. Some operations request that if, when you shut down, the tenth of an hour is anywhere between one digit and the next (perhaps even if it’s just starting to move) that you round up to the next higher tenth. In a way, it all comes out even in the end, if you fly there much at all. The advantage to a particular rule like this is that at least it removes the uncertainty about what to write down when you check back in with the dispatcher.
Jeff Pardo
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